High altitude gravity energy storage

ABSTRACT

The present invention is realized by apparatus and methods for harvesting, storing, and generating energy by permanently placing a large rigid buoyant platform high in the earth&#39;s atmosphere, above clouds, moisture, dust, and wind. Long, strong and light tethers can connect the buoyant structure to the ground which can hold it in position against wind forces. Weights suspended from the buoyant platform with cables are raised and lowered by electric winches to store and release gravitational potential energy. High voltage transmission lines electrically connect the platform to the earth&#39;s surface. Electrical energy from the high voltage transmission lines or from photovoltaic arrays on the platform can be stored as gravitational potential energy and subsequently released as electricity from generators driven from the stored gravitational potential energy and used on the platform or transmitted via the high voltage transmission lines.

CROSS REFERENCE TO RELATED APPLICATIONS:

This application is a continuation of USSN 15/611,782 filed June 1, 2017which is a continuation of 14104126 filed Dec. 12, 2013.

DESCRIPTION 1. Field

This invention generally relates to energy storage and more particularlygravity energy storage.

2. Prior Art

An area of prior art is gravity energy storage. This uses mechanicalenergy to raise a mass through a height, storing energy as gravitationalpotential energy. Energy is subsequently retrieved by lowering the mass.The most common form is pumped storage hydroelectricity which uses anelectric motor to drive a pump and transport a mass of water from a lowaltitude reservoir up a hill to a high altitude reservoir in order tostore energy. Electrical energy is subsequently recovered as water flowsback down the hill driving a turbine that drives a generator.

On a smaller scale the original “grandfather” clocks used humanmechanical energy to raise weights and store it as gravitationalpotential energy. As the weights slowly descended the recoveredmechanical energy powered the mechanical clocks.

Because of the perceived need for energy storage solutions to complementintermittent alternative energy sources such as wind and solar energythere are several new gravity storage ideas being pursued, though nonehave yet been deployed. These include using buoyancy in the ocean astaught in US patent US20100107627, various means of transporting mass asrock or gravel up a height using continuous mechanical conveyors ofcontainers and subsequently bringing the mass back down as taught inCN2307111, and giant pistons, usually made of rock as taught inUS20120085984.

As with all energy storage solutions the central problem is cost. Acommonly perceived cost goal for economic viability is $100/kWh capitalcost. There is as yet no energy storage solution that is close toachieving this goal. Most of the various forms of existing and proposedgravity energy storage also suffer from geographic constraints thatlimit their scale and/or location.

Another area of prior art is buoyant airships and balloons that float inthe atmosphere.

Balloons float freely without propulsion and are constructed from gastight flexible membranes, usually thin plastic like polyethylene. Theycontain a lighter than air gas, sometimes pressurized and sometimesunpressurized. A commonly used terminology is “zero-pressure” forunpressurized balloons and “super-pressure” for pressurized balloons.Free floating balloons of both types are typically exploited fortracking weather or for scientific purposes. The largest balloons havevolumes of about 1 million cubic meters and can float in thestratosphere at altitudes exceeding 40 km. They are fragile, carry smallpayloads and are used for one flight.

Airships have an aerodynamic shape and a means of propulsion and arecategorized as rigid, semi rigid or blimps.

Blimps, like the Goodyear blimp, use a gas tight membrane filled with apressurized lighter than air gas to provide a combination of buoyancy,structural rigidity, protection from weather and an aerodynamic shape.This means of construction combined with the limited strength ofavailable membrane materials has limited the scale of blimps to a volumeof a few thousand cubic meters. Blimps either have a means of propulsionor they are tethered to the ground. A tethered blimp lacking means ofpropulsion is usually called an aerostat. Because of their limitedvolume, all blimps and aerostats have been confined to the denser airenvironment of altitudes below 10 km in the troposphere. This is becauseas the air becomes less dense with altitude, a given volume providesless buoyancy. Blimps have heavy propulsion systems, fuel and passengeror equipment payloads all of which have to be supported by buoyancy.Aerostats lack propulsion systems, but have tethers that are at least asheavy. In both cases it has proven impractical to provide sufficientbuoyancy or a sufficiently light blimp to enable operation in the lowstratosphere.

Rigid airships are constructed with a rigid framework that providesstructural rigidity and aerodynamic shape and contain zero pressure gasbags within the rigid framework to provide buoyancy. This means ofconstruction has enabled the construction of large craft with volumesexceeding 100,000 cubic meters. Rigid and semi rigid airships have allbeen powered aircraft. Airships have only operated at altitudes wellbelow 10 km. To build airships that could operate at higher altitudeinvolves building very much larger craft. The engineering andoperational constraints of doing this combined with the lack of aneconomic or military demand have meant that this option has never beenexplored.

The earth's atmosphere in the low stratosphere in the region of 20 kmaltitude has benign weather properties over most of the earth's surfacebelow latitude 60 degrees that make it attractive for long enduranceoperation. This has been exploited by reconnaissance aircraft like theU2 and Global Hawk. Weather we are familiar with is confined to thetroposphere which extends up to an altitude from about 8 km to 12 kmwith a gradual transition to the stratosphere called the tropopause. Thehigh winds of the jet stream occur at the tropopause. There is nomoisture or clouds in the stratosphere and turbulent weather patternslike thunderstorms and hurricanes do not reach high enough to haveeffect at an altitude of 20 km. This is well illustrated by flights byU2 and Global Hawk over hurricanes for weather research. Winds aresteady and horizontal, mostly less than 20 meters per second, with smallepisodic periods in winter of a few weeks every few years where they canreach 50 meters per second due to excursions of the polar vortex whichcircles the poles in the stratosphere in winter.

The permanently benign weather properties of the atmosphere in theregion of 20 km altitude in the low stratosphere make it a distinct andseparate operational environment which enables practical long enduranceoperation as evidenced by the U2 and global hawk aircraft. The uniquelow air pressure, low air density environment requires unique aircraftdesigned to operate there. Conventional aircraft are designed to operateat lower altitudes up to around 12 km, and their aerodynamics andpropulsion systems cannot operate at altitudes around 20 km. There havebeen attempts at building long endurance high altitude airships to flyat 20 km altitude and above, but none have as yet succeeded due to thedifficult engineering challenges of limited buoyancy posed by the thinatmosphere. In the class of buoyant aircraft, only un-tethered andun-powered free floating weather and research balloons have operated inthe stratosphere.

No prior art airship or aerostat has been designed to stay aloft on apermanent basis. Endurance is measured in weeks for airships and monthsfor aerostats. They both have limited endurance and both must avoid badweather.

In summary all prior art mechanisms that float in the atmosphere havebeen relatively small scale and short endurance and almost all haveoperated in the troposphere. There have been no tethered buoyant rigidstructures operated in the atmosphere at any altitude.

Another feature of high altitude operation in the low stratosphere isthe large distance to the horizon. From an altitude of 20 km, thehorizon is approximately 550 km distant. This means that observation orcommunication technologies that are confined to “line of sight”operation can cover a wide area from this altitude. This includes activetechnologies like radars, laser and radio communications, and passiveoptical and radio surveillance. The air is clear at 20 km which enablesuninterrupted and secure laser light communication between platforms andbetween platforms and spacecraft. There have been proposals for longendurance high altitude aircraft or airships to “station keep” and actas communications and observation hubs, but the operating constraintshave proven too difficult. They would use solar energy during daylighthours with batteries storing energy for nighttime. Providing sufficientenergy to station keep in the worst case winds of around 50 m/s hasproven impractical.

Another feature of the environment in the low stratosphere is sunlightis more intense. Atmospheric scattering is much reduced due to the muchsmaller mass of air in the optical path, especially at lower sunelevation angles. This results in higher daily solar energy incident ona surface. This can exceed a factor of three or more times ground levelsolar energy at the same location depending on latitude and tracking.Also solar energy is totally predictable as it is not interrupted byweather or dust.

Photovoltaic solar energy systems use solar cells to convert solarenergy directly into electricity. The solar cells are usually connectedtogether in panels, which in turn are mounted on mechanical supports andconnected together to form arrays. Associated with the photovoltaicpanels are electrical elements such as conductors, voltage converters,combiners, fuses, relays surge protectors and inverters used to combinethe power from the collection of photovoltaic panels into a single poweroutput.

Current photovoltaic electricity systems suffer from several problems.Their high capital costs make the cost of the energy they produceuncompetitive without subsidy.

The power produced by photovoltaic panels varies by more than a factorof two depending on their geographic location. Large-scale systems inthe best sunny geographic locations also have high ancillary costs tocompensate for the long transmission distance from the system to theaverage power user.

Photovoltaic arrays need to have large entry apertures to producemeaningful amounts of power. Utility scale systems have aperturesmeasured in millions of square meters. Current systems consequentlyconsume large areas of land and significant quantities of constructionmaterials like glass and steel needed to fabricate this large aperturearray.

Weather in the form of clouds, dust, wind, rain, hail, frost and snowmake power generation unpredictable and require that structures bestrong and durable which adds significantly to their cost. Typicaldesign wind loads are around 2000 Pa and mechanical snow loads arearound 5000 Pa.

Some current large scale systems use large arrays of individuallysteered collecting elements. Robust mechanical support, motors, gears,electrical equipment etc are needed for each collector element,contributing significantly to overall cost.

The cost problem is compounded by the generally low overall energyconversion efficiency of current systems, which consequentially requiresa larger surface area and more material to produce a given power outputcompared to higher conversion efficiency systems.

There have been some proposals to attach cells to tethered aerostats togenerate power. These have all proposed current small scale aerostatstethered at relatively low altitudes in the troposphere. None of theseproposals have been reduced to practice because of practical constraintsthat make them unrealistic. At all altitudes in the troposphere, weathercan be severe and the durability of current aerostat technology is poor.The small scale of aerostats mean that they can at best only provide asmall amount of power, and many thousands would be needed to providepower at a utility scale of hundreds of mega Watts. They would need tobe spaced far apart to avoid colliding. There would be a constant needto winch them down for maintenance and to avoid weather.

SUMMARY

The present invention is realized by apparatus and methods for placing alarge rigid buoyant platform high in the atmosphere, above clouds,moisture, dust, and wind. Long, strong and light tether(s) can connectthe buoyant structure to the ground which can hold it in positionagainst wind forces. The platform can serve several uses, eitherindividually or in any combination. These uses include gravity energystorage implemented by using energy to power winches and cables to raiseweights supported by the platform buoyancy to a high altitude, thusstoring energy as gravitational potential energy and subsequentlylowering the weights recovering the stored energy. Electricity outputfrom a photovoltaic array attached to the platform can be stored on theplatform as gravitational potential energy and can also be coupled tohigh voltage transmission line(s) which connect from the platform to theearth's surface. The high voltage transmission lines can also be used todeliver energy from the ground to gravity energy storage on the platformor from gravity energy storage on the platform to the ground.

These and other objects and features of the invention will be betterunderstood by reference to the detailed description which follows takentogether with the drawings in which like elements are referred to bylike designations throughout the several views.

DRAWINGS Figures

FIG. 1 is a perspective view of a platform module designed in accordancewith the present invention.

FIG. 2A is a perspective view of a buoyant platform module with anattached tether and winch designed in accordance with the presentinvention.

FIG. 2B is a perspective view of a platform module designed inaccordance with the present invention at high altitude.

FIG. 3A is a simplified perspective view of the buoyant platform moduleshown in FIG. 1 and FIG. 2.

FIG. 3B is a perspective view of a small platform assembled from threeof the platform modules shown in FIG. 3A with an added inflated nosecone and tail section.

FIG. 3C is a perspective view of a small platform assembled from nine ofthe platform modules shown in FIG. 3A with an added inflated aerodynamicedge.

FIG. 3D is a perspective view of a small platform assembled from sixteenof the platform modules shown in FIG. 3A with an added inflatedaerodynamic edge.

FIG. 4 is a perspective view of a large platform assembled from twentyfive of the nine platform module, small platform elements shown in FIG.3C with an added inflated aerodynamic edge.

FIG. 5A is a perspective view of a platform module assembled folded flaton the ground.

FIG. 5B is a perspective view of a platform module unfolded to about 30degrees.

FIG. 5C is a perspective view of a platform module fully unfolded tovertical with cross bracing and gas bag added.

FIG. 6 is a perspective view from above of a small platform made from 16platform modules, during vertical deployment to form part of a largeplatform.

FIG. 7A is a perspective view from above of a large platform constructedfrom 81, small platforms, each small platform being constructed from 16platform modules, as a small platform is being added to the largeplatform.

FIG. 7B is a perspective view of a close up of the bottom front left ofFIG. 7A.

FIG. 7C is a perspective view of a stage in the deployment of the smallplatform.

FIG. 7D is a perspective view of a subsequent stage in the deployment ofthe small platform.

FIG. 8 is a schematic view of the mechanical and electrical elements ofa gravity energy storage system using a buoyant high altitude platform.

FIG. 9A is a perspective view of a gravity storage system using a smallbuoyant high altitude platform.

FIG. 9B is a perspective view of a close up view of a gravity storageweight shown in FIG. 9A.

FIG. 10 is a perspective view of a large platform with gravity storage.

TABLE—US—0001

DRAWINGS—REFERENCE NUMERALS

11 strut 12 cable cross brace 13 top surface structure 14 top surfacesection 15 photovoltaic panel top surface 16 interior gas bag 21tether-HV cable 22 winch 23 platform module 31 inflated tail section 32inflated nose section 33 inflated edge section 41 9 module smallplatform 61 deployment boom 62 16 module, small platform 63 in guides orrails 71 16 module, small platform 72 across guides or rails 73 largeplatform made of 81, small platforms 80 PV panel array 81 Electricalpower conversion 82 electrical motor-generator 83 motor-gearbox shaft 84gearbox-brake 85 gearbox-reel shaft 86 gravity cable reel 87 gravitycable 88 gravity storage weight

GLOSSARY

The specification uses several standard definitions throughout to avoidambiguity. These related definitions are tied to specific aspects of thedescription.

Platform module: the standard and smallest unit of platformconstruction.

Small platform: An assembly of platform modules.

Large platform: An assembly of small platforms.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of a buoyant platform module 23 designedin accordance with the present invention. It consists of a rigidframework formed from struts 11, top surface 13 and cross bracing cables12. The interior of the framework holds a gas bag 16, which contains thebuoyancy gas, commonly hydrogen or helium. The top surface 13 isassembled from smaller structural sections 14. The top surface of eachstructural section 14 can support an array of photovoltaic panels 15that can either partially or completely cover the top surface 13. Thephotovoltaic panels 15 are connected electrically with wires, DC-DCvoltage converters, combiners and electricity distribution hardware toprovide high voltage (HV) power output from the platform. This HV outputcan be AC or DC. In this embodiment when fully assembled, the overallstructure is a rigid cross braced cube. The length of the cube is in theregion of 100 meters. The dimensions are set by the buoyancy availableat the design altitude. The module buoyancy supports the mass of themodule and the wind loads. The buoyant platform module 23 described canbe used as a module in a modular construction system or method used tobuild larger buoyant platforms from assemblies of modules. Forsimplicity in the description we subsequently refer to these cubicbuoyant platform modules as platform modules. Embodiments are notrestricted to cubes, and other geometric forms are feasible. Aparticular variant has a height that is different than the width. Thisallows a simple change in vertical strut length and gas bag height toprovide different buoyancies. This allows embodiments with differentpayloads or operational altitudes to be easily constructed.

Equipment and materials need to operate within the environmentalconstraints of the low stratosphere. Air pressure is about 8000 Pa whichaffects buoyancy and the breakdown voltage. The air temperature isaround −60 degrees Celsius, and the ozone concentration is around 2.8ppm. These affect the choice of materials, particularly plastics thatmay become more brittle or suffer damage. The struts and top surface arelightweight, rigid truss frameworks, typically formed from aluminum. Thegas bag is typically a thin plastic membrane. A commonly used materialis polyethylene film around 25 microns thickness. The membrane may be alaminate or co-extrusion of several plastic and metal materials toprovide properties such as low buoyancy gas permeability, protectionfrom ozone, weld-ability and strength.

Photovoltaic panels are of lightweight construction, typically weighingabout 2 kg per square meter or less. Various photovoltaic celltechnologies can be employed including commonly used crystalline andpolycrystalline silicon. Given the predominance of direct solarradiation in the low stratosphere, concentrating photovoltaic panelsthat need to track the sun may benefit. Photovoltaic panel materialsneed to handle the cold and the UV, particularly the cell encapsulantmaterial. Silicone is one good choice. Compared to photovoltaic panelson the ground, the need for water based weather protection is reduced asthere is no water in the low-stratosphere operating environment. Groundbased photovoltaic panels as well as handling water based weathering,also have to handle snow loads of around 5000 Pa, hail, regular washing,and maximum wind loads of around 2000 Pa. In contrast, in thelow-stratosphere there is no hail, snow, or significant dust, andmaximum wind loads are about 125 Pa to 150 Pa, so photovoltaic panelscan be simpler and less robust photovoltaic panels are highly reliable,and the absence of water based weather degradation and the low operatingtemperature will enhance this reliability in the low-stratosphere.

Arrays of photovoltaic panels 15 can be formed in the same ways they areon the ground. The simplest form is a flat array covering the surface.Single fixed axis, one axis tracking and two axis tracking are all alsopossible. Because the structural array has a cost per unit area,optimizing the area usage is more important than with ground basedphotovoltaic arrays, and is similar to ground based commercialphotovoltaic systems on roofs that want to optimize the electricitygenerated for the roof area. As with ground based photovoltaic arrays,detailed cost analysis based on the cost of photovoltaic panels, theadditional costs of tracking apparatus and the geographic locationdetermine what is the most cost effective array form to deploy.

Embodiments of platform modules may not cover the entire surface with aphotovoltaic array 15, or even any photovoltaic panels. As part of alarger platform they may serve other roles, such as providing active andpassive fire safety, by providing fire suppressants or acting as a nonflammable fire break.

They may also support other payloads such as winches and weights toimplement gravity energy storage. They may also support wireless orlaser communication systems for communication with the ground, space, orother stratospheric platforms. They may also support radar systems foruses such as monitoring weather, air traffic control and military uses.Other military uses include use as a weapons platform carrying missiles,direct energy laser weapons or drone aircraft. They may also supportobservation systems such as space telescopes and ground monitoring. Theymay also support scientific payloads.

Many of these uses are enabled or enhanced by the permanence of theplatform, the large payloads that can be carried and the large andpermanent electrical power that is available. Payloads of hundreds oftonnes are possible and power of many hundreds of mega watts areavailable. Night time power can be provided from electricity storage onthe platform from batteries or gravity storage or from the electricitygrid on the ground.

FIG. 2A shows a different perspective view of the platform module shownin FIG. 1. The module is the same as in FIG. 1 with the addition oftether/HV cable 21 and winch 22. The module is shown during deployment,floating at about 100 meters altitude and being held by the tether/HVcable 21. The gas bag 16 is shown in a partially inflated condition. Asthe module rises in altitude and atmospheric pressure reduces, the gasbag expands. Sufficient buoyancy gas is added to the gas bag such thatit is nearly fully inflated at a nominal operational altitude of about20 km. Throughout this specification reference to 20 km altitude ismeant to be interpreted as approximately 20 km. The low stratospherevaries in altitude and the precise operational altitude of platformswill vary by location, and perhaps by season. The tether may include theHigh Voltage (HV) cable that carries power from the module to the groundor from the ground to the module when necessary, such as during thenight. The tether is strong and lightweight and typically made from anaramid fiber such as Kevlar. For efficiency and simplicity the highvoltage is typically direct current (HVDC). The combination of highvoltage transmission and aluminum conductors keeps the HV transmissioncable lightweight.

FIG. 2B shows the platform module deployed to a high altitude, on itsway to operational altitude. Winch 22 is playing out the cable 21. Theplatform 23 is shown in detail in FIG. 1.

Compared to prior art airships and aerostats, a novel and necessaryfeature of the platform module 23 described above is the scale. Thebasic 100 meter cube module 23 has an approximate buoyancy volume of1,000,000 cubic meters, which far exceeds the 200,000 cubic meters ofthe Hindenburg, still the largest airship ever built. The scale isnecessary because the air at 20 km altitude is very thin and a ratio ofvolume to top surface area of about 100 is needed to carry thestructural weight and HV cable and platform wind loads resisted by thetether. Only flimsy disposable balloons for science research in the highstratosphere have approached 1,000,000 cubic meters in volume.

The rigid framework provides the support structure for the photovoltaicpanels and carries the wind induced loads. A simple zero pressure gasbag needs no control mechanisms to adjust for pressure changes and as anexample the buoyancy gas leakage for a gas bag of these dimensionsconstructed with 25 micron aluminized PET membranes is considerably lessthan 1% a year. For platforms with a design life of 20 to 30 years,buoyancy gas may not have to be replenished for the life of theplatform. Endurance measured in decades is more accurately described asa design life, a term normally applied to structures such as buoyantocean platforms or bridges.

FIGS. 3A, 3B, 3C and 3D show small platforms assembled from platformmodules 23. FIG. 3B shows a small platform constructed from threemechanically connected platform modules 23 with an attached inflatednose 32 and tail 31. The tether 21 attaches to each platform module todistribute the mechanical load and combine the HV power output from eachplatform module. FIG. 3C shows a small platform constructed from ninemechanically connected platform modules 23 with an attached inflatedrounded aerodynamic edge 33. FIG. 3D shows a sixteen platform module 23small platform with an inflated rounded aerodynamic edge 33. Inflatededge 33, nose 32 and tail 31 are each gas-tight, light-weight fabriccontainers filled with pressurized gas, most commonly air. The gaugepressure might be in the region of 300 Pa to 500 Pa. Commonly usedfabrics are laminates of materials that provide various properties. Anexample of such a laminate might have an exterior layer of polyvinylfluoride film for protection from weather, a layer of polyester fabricfor strength and an inner layer of polyurethane film for gas tightness.

Each of these small platforms are assembled on the ground and thendeployed to 20 km altitude similar to as shown in FIG. 2B. FIGS. 3A, 3B,3C and 3D show small platforms deployed as small stand alone powerplants. The sixteen platform module, small platform shown in FIG. 3Dmeasures about 400 meters on a side. The difficulty of manufacturing anddeploying larger assemblies of platform modules 23 leads to the methodshown in FIG. 4.

FIG. 4 shows a large platform composed of 25 of the small platforms,each constructed from 9 platform module assemblies 41 shown in FIG. 3C,mechanically connected together and deployed at 20 km altitude. As canbe seen each small platform 41 has its tether/HV cable 21 attached. Eachsmall platform 41 is individually assembled on the ground and thendeployed to 20 km altitude using its own tether/HV cable 21 and winch22. On its ascent, each small platform 41 is guided by attached boomsthat connect to adjacent tethers of previously deployed small platforms41 and use them as guides and for horizontal support. This ensures thatthe deploying small platforms 41 do not collide with adjacent tethersand also the adjacent tethers provide mechanical support to help thedeploying small platforms 41 resist wind loads. It also ensures thatsmall platforms 41 can be simply and accurately guided into their matingposition within the larger platform where they can be mechanicallyconnected to become part of the larger platform structure. This processis reversed to bring small platforms 41 down to the ground formaintenance or repair. The large platform only ever exists at 20 kmaltitude and only small platforms 41 are handled on the ground. Thisterminology distinguishing between small and large platforms isstandardized in this description. Large platforms are always assembledfrom small platforms at altitude in the low stratosphere. Smallplatforms are always assembled on the ground from platform modules.

The large platform shown in FIG. 4 has 25 tethers. This providesdirectional stability and redundancy. Compared to a single smallplatform 41, the large platform has 25 times the buoyancy, but only fivetimes the frontal area, and so is deflected far less in high winds. Aslarge platforms grow, this effect continues and platforms become morestable and redundant.

FIGS. 5A, 5B, and 5C illustrate part of a method to construct a platformmodule 23. FIG. 5A shows the module structure assembled folded flat onthe ground. This allows construction and assembly to occur convenientlyat ground level. When the assembly of the structure including all andelectrical assembly is complete and tested at ground level, thestructure is unfolded to its final cubic configuration. Hinged joints 51at the eight vertices of the cube connect the structural elements andenable the unfolding from flat to cubic. The forces used to raise thestructure could be cables and pulleys which are not shown. FIG. 5B showsan intermediate position as the module is unfolded. FIG. 5C shows thefinal position where the joints have been rigidly connected and thehinges are locked and no longer operate. The cross bracing has beenadded and the gas bag is shown added and inflated. This method ofdeploying the gas bag within the rigid protective structure greatlysimplifies the deployment of such large fragile elements that couldeasily be damaged by simple contact with the ground as a result of asudden gust of wind.

The method shown can be easily extended to unfold multiple joinedplatform modules from a folded flat position using hinges at thevertices of each cube. These small platforms can then be deployed tohigh altitude and joined to form a single large multi element structureusing the method described in the description for FIG. 4 and FIG. 7.

Another embodiment of the folding method described would break thevertical struts with additional hinges and fold the struts under theplatform surface.

FIG. 6 shows a perspective view of a small platform 62 assembled from 16platform modules 23 as small platform 62 is deploying to altitude. Thisillustrates the method of deployment described in the description forFIG. 4. For clarity only the nearest neighbor tethers 21 are shown. Thisshows the guiding and supporting booms 61 attaching the small platformto the adjacent tethers 21. As a winch plays out the tether attached tosmall platform 62 the small platform rises or falls vertically in acontrolled manner accurately positioned and restrained horizontally bythe booms 61.

During initial assembly of the large platform, there are few deployedsmall platforms and supporting tethers for guidance. Unique deploymentmethods are required using additional cables to help guide and supportdeploying platforms 41 or 62.

FIGS. 7A, 7B, 7C and 7D show various perspective views describingadditional stages of small platform 71 deployment described in thedescription of FIG. 6. In all views, most of the tethers 21 are omittedfor clarity.

FIG. 7A shows a perspective view of a large platform 73 floating ataltitude. An assembled small platform 71 is shown at the bottom, aboutto be deployed to altitude.

FIG. 7B shows a close up perspective view of small platform 71 andelements that enable its deployment. In-rails or guides 72 are used tohelp guide and transport small platform 71 through the space between thetethers 21 to the location of the across-rails or guides 63.

FIG. 7C shows a perspective view of a stage of deployment of smallplatform 71 when it has completed its transport along in-rails 72. Inthis embodiment across-rails 63 are shown above in-rails 72, but theycould be at the same level as in-rails 72, with either or both at groundlevel or elevated using the tethers 21 as support.

FIG. 7D shows a perspective view of small platform 71 after it hastransferred via across rails 63 to the desired tether 21 location forvertical transport. The vertical stage in deployment is shown in FIG. 6,where booms are deployed and attached to adjacent tethers 21 andaerodynamic edges are inflated.

FIG. 8 shows a schematic view of elements attached to a small platform62 that implement gravity energy storage. PV panel array 80 iselectrically connected to electrical conversion and control block 81.Electrical conversion and control block 81 is electrically connected toHV cable-tether 21 and electrical motor generator 82. Electrical motorgenerator 82 is mechanically connected to gearbox-brake unit 84 withshaft 83. Gearbox-brake unit 84 is mechanically connected to cable reel86 with shaft 85. The combination of electrical motor generator 82,gearbox-brake unit 84, shaft 83, gearbox-brake unit 84, cable reel 86and shaft 85 make a winch. Cable reel 86 holds long cable 87 which isattached to gravity storage weight 88. Cable 87 is made from strong andlight material suitable for storing on reel 86. Example materials arearamid fibers such as Kevlar or ultra high molecular weight polyethylene (UHMPE) such as Dyneema.

FIGS. 9A and 9B show perspective views of a small platform thatincorporates the gravity energy storage elements shown in FIG. 8. The PVarray 80 is on the top surface of small platform 62. The electricalconversion and control block 81, and winch elements are attached tosmall platform 62 inside the framework and are not visible in theseviews. In other embodiments these elements could be deployed on aplatform suspended below small platform 62 as a means to evenlydistribute the mechanical load. Gravity storage weight 88 is shown inFIG. 9A and in more detail in FIG. 9B. This illustrates the relativelysmall size of weight 88 relative to small platform 62. The gravitystorage weight 88 can be implemented as a dense solid or as a granularsolid such as sand or a liquid such as water held in a container. Waterand sand have the advantage in that they are easily jettisoned in anemergency such as a cable breaking, minimizing the hazard on the ground.

FIG. 10 shows a perspective view of a large platform 73 assembled ataltitude from small platforms 62, each incorporating gravity energystorage. This illustrates the modular, distributed and redundant natureof high altitude gravity energy storage, with many gravity storageweights 88 and their attendant winches and cables. Systems can growincrementally to very large storage capacity, and failure of one elementhas little impact on overall operation.

The magnitude of gravitational potential energy is equal to the massmultiplied by the force of gravity multiplied by the height. The gravityenergy storage system described is unique in that it exploits buoyancyin the atmosphere to support a large mass and the height ofapproximately 20 km is much larger than the hundreds of meters height ofthe prior art. Platforms described can scale to support a mass of manythousands of tons, which when multiplied by the 20 km altitude resultsin much larger potential energy gravity storage than the prior art.

Operation

The small platforms floating tethered in the low stratosphere shown inFIGS. 3B, 3C and 3D, and the large platforms shown in FIGS. 4, and 7Aoperate to produce photovoltaic electricity in the manner of prior artphotovoltaic power plants on the ground. The electrical elements,including photovoltaic panels, wires, combiner elements, DC-DCconverters, DC-AC inverters are the same but have to be designed tooperate in the unique environment in the low stratosphere, which iscolder, has thinner air with low buoyancy and a low breakdown voltage,has more ozone and has higher intensity solar radiation with more UV.They also can be optimized for operation in the low stratosphere, as isthe case for photovoltaic panels which don't have to handle water basedweather and are exposed to much lighter mechanical loads from wind andthe absence of snow, hail and ice.

When deployed and operating in the low stratosphere, the small platformslike those shown in FIGS. 3A, 3B, 3C and 3D, and the large platformsshown in FIGS. 4, and 7A operate passively to resist wind loads andatmospheric changes. The structures move under wind loads and buoyancyprovides horizontal reaction forces via the tethers that counteract windforces. The volume of gas in gas bags expands and contracts within therange of pressure changes at the deployed altitude.

Tether/HV cables 21 are also subject to extreme wind speeds in thetroposphere, but their narrow diameter ensures that the aerodynamicloads are small in comparison to the worst forces on the buoyantplatform and these forces are also counteracted by platform buoyancyreaction forces.

Operation also includes deployment and maintenance and repair. Thephysical scale of the buoyant structures shown in FIGS. 3A, 3B, 3C, 3D,4, and 7A is larger than any prior art buoyant apparatus. The apparatusand methods that enable construction and deployment of such large scaleobjects is by definition new. The design of platform modules 23 isdescribed in the description of FIG. 1 and FIG. 2. Each platform moduleis a fully functional self contained array of photovoltaic panels,electrical systems, structural systems, buoyancy systems and optionalother use systems. This greatly facilitates the construction of largerstructures from assemblies of these modular elements which simply needto be mechanically connected. The method of construction of platformmodules 23 shown in FIGS. 5A, 5B, and 5C that enables all assembly workto occur at ground level greatly simplifies the construction process.The addition of appropriate hinges 51 at the vertices of platformmodules 23 enables this form of construction.

The area of flat land needed along with logistical and operationaldifficulties make it impractical to construct, deploy and maintain verylarge platforms from the ground. The method of small platform deploymentshown in FIGS. 6, 7A, 7B, 7C, and 7D using guide rails 63 and 72 movesthe assembly process of large platforms above the ground and then to thelow stratosphere. This process of deployment, maintenance and recoveryof small platforms that can be reasonably assembled and repaired on theground and joined together only in the low stratosphere makes theconstruction of large platforms possible. The large platforms only everexist as such in the stratosphere. The assembly area on the ground issmall and if guide rails 63 and 72 are elevated, the ground under thelarge array is undisturbed during operation and maintenance. Over timewith proven safety and reliability, large platforms could become verylarge as the only additional impact on the ground with large platformgrowth is winches and HV distribution.

The impact of HV distribution on the ground can be reduced by using thetethers as support “towers” from which HV cables can be suspended highabove the ground, perhaps at several hundred meters altitude. Thedistance between tethers is similar to the distance 18 between HV towersin current art HV power distribution, so cables and equipment couldeasily be adapted.

With low leakage gas bags and highly reliable and redundant photovoltaicpanels and electrical systems, it is likely that platforms will stayaloft for years before maintenance or repair is required. Whennecessary, the method and apparatus shown in FIGS. 6, 7A, 7B, 7C, and 7Dallows small platforms to be winched down and repaired or replaced whenit is operationally convenient.

Each small platform in the large platform can have its own tether. This,as well as allowing for maintenance and repair provides tetherredundancy and ensures that mechanical loads on the platforms are evenlydistributed. This in turn reinforces the modular structural design asmechanical loads are constant or reduce as the large platforms grow.

As shown in FIG. 8, FIG. 9A FIG. 9B and FIG. 10, each small platform 62in a large platform can implement gravity energy storage. In operation,energy is stored by using electrical energy to raise weight 88 usingmotor/generator 82 and cable 87. The electrical energy to drive themotor generator can come from a PV array 80 on the platform or from theground via the platform HV transmission cable-tether 21. In this manner,energy storage is not necessarily tied to PV electricity generation onthe small platform 62. Stored gravity energy is recovered by allowingweight 88 to fall at a controlled rate, with attached cable 87mechanically driving motor generator 82, thereby producing electricity.This electricity can be consumed by equipment on the small platform 62and/or converted to high voltage DC by electrically connected powerconvertor controller block 8 land transmitted down HV transmissioncable-tether 21.

The normal usage model envisaged is to store energy generated duringdaylight for use during darkness, thus following a dailycharge-discharge cycle. However, storage and generation are notnecessarily tied to a daily cycle, and usage can vary. For exampleenergy stored early in the day may be recovered later, still duringdaylight. If small platform 62 is at 20 km altitude, and the weight israised and lowered through the full 20 km distance, the energy stored inweight 88 is approximately 54 Watt hours per kilogram. A representative500 tonne weight would store approximately 25,000 kilo Watt hours ofgravitational potential energy.

Though not shown or discussed platform modules have systems to handlestatic electricity and lightning. There are instrumentation systems tomonitor the electrical, structural, buoyancy systems, gas leakage, fireenvironmental pressure, temperature, sunlight and other variables. Thereare control systems to handle system deployment, fire and electricalsafety systems.

Also not shown are all uses of the small or large platforms forcommunications or observation for civilian and military use or as aweapons platform for military use. These uses could be added to powerplatforms or be provided on platforms not primarily designed to provideelectricity.

Advantages

The gravity energy storage system enabled by the buoyant platformsdescribed has many advantages over prior art energy storage systems. Theround trip efficiency ratio of electrical energy out to electricalenergy in can easily exceed 90%. With regular maintenance, round tripefficiency will stay constant for the life of the platform which couldexceed 50 years or more. Batteries in contrast diminish in energystorage capacity with use and have a short life.

The cost of the gravity energy storage is potentially considerably lessthan $100/kWh capital cost which can result in an added cost of lessthan $0.03/kWh additional cost for stored electricity. This is far lowerthan any current energy storage technology and sufficiently low to makeelectricity supplied from storage competitive with electricity generatedfrom burning fossil fuels.

The buoyancy needed to provide gravity storage is similar to thebuoyancy needed to support PV electricity generation, which makes themvery compatible for providing a unified solution that delivers acontinuous, uninterrupted and reliable supply of low cost electricityfrom solar energy.

As described, gravity energy storage can be provided in modularincrements of Mega Watt size as opposed to some technologies like pumpedstorage that must be developed on a much larger scale. These modularincrements can be aggregated to provide very large scale energy storageof many Giga Watts, far beyond any currently deployed solution.

There is no geographic constraint on gravity energy storage asdescribed, compared to pumped hydro electric storage, compressed airstorage and other large scale storage technologies. Gravity energystorage is co-located with PV energy generation and can scale and growwith PV generation in a balanced manner.

In addition to PV power generation and gravity energy storage, becauseof the benign weather free environment with abundant reliable solarpower and clear visibility to space and a horizon exceeding 550 kmradius, many synergistic new uses, either alone or in combination, areenabled and supported by large rigid buoyant tethered platforms floatingin the low stratosphere. The scale of the power and payload provided andthe permanent nature of the platforms enable not just new communicationuses, but very large scale, very reliable, very high bandwidth verysecure communications networks. For example the whole land area of themainland united states could be covered with approximately fourteenplatforms. Each platform could communicate with neighboring platformsvia laser or radio, with spacecraft with laser or radio, and with theground via secure fiber optic cables. If platforms are deployed near allmajor urban areas to provide photovoltaic electricity, there will beseveral hundred platforms in the US and communications networkssupported by them would be highly redundant. The coverage area from eachplatform for radio communications would match that of thousands of cellphone towers, with fewer dead zones. The line of sight visibility couldenable the use of higher frequency radio bandwidth.

Because of exposure to more solar energy and the cold operatingenvironment that increases the efficiency of many solar celltechnologies, the photovoltaic electric power output is many times thatof a same sized prior art ground system. This means the cost of theelectricity produced is lower.

Power output is high at high latitudes, and is not affected by clouds,dust, or bad weather. This is of particular benefit to normally cloudynorthern and mid latitude locations where most large urban areas arelocated.

The combination of geographic flexibility and power generation withoutthe need for any fuel provides a secure and clean energy system.

Power in the form of electricity can be provided at any point on theearth's surface, where the definition of surface includes the entiresurface, including all land and oceans. Offshore platforms, or platformsthat straddle land and ocean could be a particularly convenient in somelocations. electricity could be provided near mines, allowing convenientprocessing without transportation of bulk ores.

The small amount of land area needed means that systems can be locatedvery near existing power plants, or existing transmission anddistribution networks, which reduces or eliminates the need for newelectricity transmission infrastructure.

Systems can scale to very large size. This means that fewer platformsare needed which reduces the impact on aircraft and airspace.

Because the land and environmental impact is small, the platforms usecommonly available materials that have no resource or manufacturingconstraint and the generated electric power is low cost, the systems canscale to provide all needed energy.

The manufacture of synthetic fuels for transportation and long termenergy storage using the cheap electricity from the platforms provides acomplete energy solution for all current uses.

Energy systems that do not put carbon dioxide into the atmosphere arehighly desirable. Currently all alternative energy systems suffer frommajor problems: [0113] 1) They are very costly to build [0114] 2) Theyare unreliable providers of electricity due to intermittent weathereffects, and so need backup generation using alternate energy sourcessuch as natural gas.

3) They need large additional energy storage and transmissioninfrastructure investments. [0116] 4) The most abundant energy islocated far from users, again requiring large transmissioninfrastructure investments. [0117] 5) They require large areas of landwhich increases their environmental impact and limits their use to areaswhere both energy and land are available.

This new system has the benefit of not producing carbon dioxide and hasnone of these problems. The bottom line is clean secure energy can beprovided at much lower cost and minimal environmental impact.

The benefits of suspending an array in the stratosphere are thereliability of the energy source, the higher incident energy density,and the benign stable calm low wind weather free environment thatenables permanent tethering. These benefits come at the price of loweratmospheric density, which means less buoyant lift and a consequent needfor a large lightweight structure.

The modular manufacturing and deployment methods described greatlyreduce cost, improve quality, and speed construction. It is envisagedthat when production is mature, complete utility size electricitygenerating facilities could be operational in less than a year frombreaking ground. This compares with current technologies which requirethree to five or more years to construct.

Although the present invention has been described in terms of a firstembodiment, it will be appreciated that various modifications andalterations might be made by those skilled in the art without departingfrom the spirit and scope of the invention.

The invention should therefore be measured in terms of the claims whichfollow.

1. A large rigid buoyant rectangular platform residing in the lowstratosphere having a tether to the ground, said large rigid buoyanttethered rectangular platform comprising multiple small rectangularplatforms positioned in a rectangular configuration forming said largerectangular platform, said large rectangular platform comprising a ratioof buoyancy to a frontal area greater than a ratio of buoyancy tofrontal area of a small rectangular platform.
 2. A large rigid tetheredbuoyant rectangular platform of claim 1, wherein said multiple smallrectangular platforms are squares.
 3. A large rigid tethered buoyantrectangular platform of claim 1, said large platform comprising asquare.
 4. A large rigid tethered buoyant platform of claim 1, saidplatform tethered to the ground by multiple tethers.
 5. A large rigidtethered buoyant platform of claim 4, said platform tethered by multipletethers to a cable, said cable tethered to the ground.
 6. A large rigidtethered buoyant platform of claim 1, comprising a tether comprising ahigh voltage cable.
 7. A large rigid tethered buoyant platform of claim1, comprising a tether comprising a fiber optic cable.
 8. A large rigidtethered buoyant platform of claim 7, wherein said tether comprisingsaid fiber optic cable runs from said platform in the low stratosphereto the ground.